Idealized simulations of the effect of Taiwan topography on the tracks of tropical cyclones with different steering flow strengths

Abstract

This is the fourth part of a study on the effect of topography on tropical cyclone (TC) tracks. In previous parts, idealized simulations were conducted using the Weather Research and Forecasting model to investigate the underlying mechanisms for the upstream deflection and the local and remote topography effect on TCs with different sizes. None of these simulations included any background flow. In this part, uniform easterly steering flows of 5 and 10 m s−1 are introduced to study the track changes under such conditions.Lee vortices are generated downstream of the Taiwan topography by the steering flow. The TC–terrain‐induced gyres found in the previous parts also develop when the TC circulation begins to interact with the topography and subsequently dominate over Taiwan. The cyclonic gyre merges with the cyclonic lee vortex induced by the steering flow and the anticyclonic gyre is advected cyclonically around the TC centre, which later deflects the TC. Because the circulation at the outer radii of a TC ‘competes’ with the steering flow, a smaller TC under a stronger steering flow has to move closer to Taiwan in order to generate such gyres. This ‘delayed’ generation of the gyres then leads to the ‘delayed’ onset of the northward deflection for such a TC. For a large TC, the circulation at its southern part is weakened by the steering flow so that the track of the anticyclonic terrain‐induced gyre is changed, which results in a larger track deflection of the TC.